1. Field of the Invention
The invention relates to a telephone set supplied by line direct voltage through two wires of a subscriber line, the set comprising an active line voltage stabilizer circuit connected between the aforesaid wires of the subscriber line, the circuit comprising two output terminals with there being between them a direct voltage which is independent of the line direct current, a first terminal of which being connected to a first of the aforesaid wires via a resistor substantially conducting the whole line direct current, with there being connected between this first terminal and the second wire a series arrangement composed of a capacitor and a further resistor, the capacitor connections of which constituting a direct voltage supply output.
The invention likewise relates to a line voltage stabilizer circuit suitable for use in such a telephone set.
2. Description of the Related Art
Such a telephone set is known from the public application report "Application of the transmission circuit TEA 1060 and the supply circuit TEA 1080", report number ETT 8506, N. V. Philips' Gloeilampenfabrieken, The Netherlands, more specificially page R23, FIG. AP1.
The stabilizier circuit shown in the above publication comprises an operational amplifier connected to the base of an output transistor. The emitter of this output transistor, forming the first output terminal, is connected to a first wire via an emitter resistor substantially conducting the whole line direct current. The collector of the output transistor is connected to the remaining wire. A resistor (referred to as R55 in the above-mentioned figure) is connected to an input of the operational amplifier. The other side of this resistor, forming the second terminal of the stabilizer circuit, is connected in the aforesaid publication to the second wire. The latter connection achieves the direct voltage on the subscriber line to be set to the desired value by the stabilizer circuit. It should be observed that details about achieving the desired line direct voltage are described in the Dutch patent application No. 8400508, laid open to public inspection, corresponding to U.S. Pat. No. 4,680,789, assigned to the present assignee.
In modern electronic telephone sets it is desirable to have a direct voltage supply connection from where additional equipment (such as for example DTMF dial dircuits or memories) can be supplied by power derived from the subscriber line. Such a supply connection should preferably meet the following three requirements:
1. within the working area of the telephone set the supply voltage must not depend on the magnitude of the DC line current;
2. the AC impedance of the telephone set, as seen from the subscriber line, must not be affected by the presence of the supply circuit;
3. when deriving direct current from the supply connection its direct voltage is only allowed to show a slight decrease, so its output impedance has to be low.
The stabilizer circuit shown in the aforesaid application report comprises a direct voltage supply connection for supplying additional equipment. Thereto a series circuit of a capacitor and a resistor is inserted between the first output terminal of this stabilizer circuit (thus the emitter of the output transistor) and one of the wires of the subscriber line; the capacitor connections will then form the supply connection. The object of the series resistor is to avoid the series arrangement forming too low an impedance for the audio signal to be transmitted by the telephone set. This implies that the series resistor must not have a much smaller value than the line terminating impedance. A customary value for the line terminating impedance is 600.OMEGA.. As the output impedance of the supply connection is mainly formed by the aforesaid series resistor, this circuit does not meet the third requirement of having a low output impedance.